Some examples of small telephone systems have been generally referred to as key telephone systems. Traditionally a key telephone system is provided by extensive telephone line and control lead wiring between key telephone sets. Each key telephone line extends to a telephone exchange. Each of the telephone sets includes a plurality of push button switches or keys, each for connecting the telephone set to a particular telephone line among a plurality of telephone lines routed to the key telephone set. The switching function of line selection is mechanically provided and distributed among the key telephone sets. Any features in addition to plain ordinary telephone service (POTS) must be added on a per line basis. The primary advantage of these systems is economy with small size. However, if such a system is required to expand along with the organization it serves, over a time it eventually becomes more expensive on a per line and feature basis than a private branch exchange would be. Key telephone systems are also characteristically of the analog signal type, and therefore are impractical to interface with an ISDN as will likely be desired by business customers in the near future.